1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and a process cartridge therefor, which are applicable to copiers, facsimiles, laser printers, direct digital platemakers, etc.
2. Discussion of the Background
An inorganic photoreceptor formed from selenium, zinc oxide or cadmium sulfate was mostly used as an electrophotographic photoreceptor for use in an electrophotographic image forming apparatus applicable to a copier and a laser printer, and now an organic photoreceptor is more widely used than the inorganic photoreceptor because of less damaging the global environment, its low cost and design freedom.
The organic photoreceptor is classified to (1) a uniform single-layered photoreceptor wherein a photoconductive resin typified by polyvinylcarbazole (PVK) or a charge transfer complex typified by PVK-TNF (2,4,7-trinitrofluorenone) is formed on an electroconductive substrate; (2) a dispersion single-layered photoreceptor wherein a resin including a dispersed pigment such as phthalocyanine and perylene is formed on an electroconductive substrate; and (3) a multilayered photoreceptor wherein a photosensitive layer formed on an electroconductive substrate is functionally separated to a charge generation layer (CGL) including a charge generation material such as an azo pigment and a charge transport layer (CTL) including a charge transport material such as triphenylamine.
The multilayered photoreceptor includes a photoreceptor including a CTL on a CGL and a photoreceptor including a CGL on a CTL. The former is typically used and the latter is occasionally called a reversely-layered photoreceptor. Particularly, the multilayered photoreceptor has an advantage of having higher sensitivity and design freedom for higher sensitivity and durability. Therefore, most of the organic photoreceptors are multilayered.
As an importance of manufacturing in consideration of global environmental protection increases recently, a photoreceptor is required to change to a machine part from a supply product (a disposable product). Therefore, the photoreceptor needs to have a long life, and a protective layer is typically formed on a photosensitive layer thereof.
As a toner for developing in electrophotography, a polymerized toner, a spheric toner, a toner having a small particle diameter (approximately 6 μm or less) and a low-temperature fixable toner are mostly used to less damage the global environment when the toner is produced, to consume less electricity and to produce high-quality images.
On the other hand, as a method of removing a toner remaining on a photoreceptor, a method of scraping the toner off from the surface of a photoreceptor with the edge of a cleaning blade formed of an elastic body such as a rubber, which is contacted thereto upon application of pressure, is widely known.
The smaller the particle diameter of a toner, the more difficult to remove the toner from a photoreceptor. Particularly when a cleaning blade is used, the smaller the diameter, the more toners scrape therethrough. Therefore, the a cleaning brush is often used together therewith.
The cleaning brushes are broadly classified to leiotrichous brushes as disclosed in Japanese Laid-Open Patent Publications Nos. 7-155222, 6-236134, 9-22155 and 11-167224; and loop brushes as disclosed in Japanese Patents Nos. 2793647 and 2619424. In addition, a combination thereof as disclosed in Japanese Patent No. 28688539 is also available.
The brushes are formed of a nylon fiber, a polyester fiber, an acrylic fiber, a carbon fiber, etc.
Cleanability of the leiotrichous brushes deteriorates because the fibers thereof droop as they are used. Although the loop brushes have better cleanability than the leiotrichous brushes, the cleanability thereof also deteriorates because toners go into loop holes and adhere thereto to clog the holes.
A combination of the cleaning brush and blade is effectively used to enhance the cleanability.
Cleaning blades formed of a plate substrate made of aluminum or iron a rectangle rubber sheet having a thickness of from 1 to 5 mm is fixed on are suggested in Japanese Laid-Open Patent Publications Nos. 8-248851, 2001-242758and2002-055582.
The rubbers have a reaction elastic modulus of from 30 to 60%. However, the rubbers have a different rubber hardness (JIS-A hardness), from 50 to 90°, i.e., the rubbers have a hardness of 60 to 85°, 50 to 90° and 50 to 80° in Japanese Laid-Open Patent Publications Nos. 8-248851, 2001-242758 and 2002-055582 respectively.
A polyurethane rubber used as a blade material particularly having high durability, very good adhesiveness to an organic photoreceptor and high cleanability is preferably used to clean a residual powder such as a toner.
However, when an additive included in a toner, particularly a wax or a particulate inorganic oxide, release and separate therefrom, and adhere to the surface thereof, such an additive is difficult to remove with a conventional cleaner from a photoreceptor and remains thereon, resulting in so-called filming wherein the additive is firmly fixed thereon upon application of pressure and frictional heat of the blade.
Further, the particulate inorganic oxide is buried on the surface of the photoreceptor, which causes accumulation of the toner, additives thereof or a paper powder of a transfer paper, resulting in filming.
Such filming impairs removal of a discharge product and a paper powder of a transfer paper sticking to the surface of the photoreceptor, and a moisture in the air also sticks thereto to decrease the resistance thereof, resulting in serious problems such as deterioration of image resolution and image distortion.
When a cleaner combining the cleaning blade and brush, the cleaning brush forcibly removes foreign particles difficult to remove with the blade, but a thin film firmly fixed on the surface of a photoreceptor, such as filming, can scarcely be removed.
To remove the filming, the surface of the photoreceptor is slightly chipped off with the cleaning blade and brush, which are adjusted to do so. However, the photoreceptor is abraded due to repeated use and noticeably has a short life.
However, as mentioned above, due to recent requirements for higher durability of the photoreceptor, the protective layer having good abrasion resistance is typically formed on the photosensitive layer thereof.
Although the protective layer improves the abrasion resistance of a photoreceptor and extends the life thereof, it is difficult to remove the filming by slightly chipping off the surface of the photoreceptor.
Because of these reasons, a need exists for an image forming apparatus capable of keeping producing high quality and resolution images using a heavy duty photoreceptor having high abrasion resistance, wherein the filming is prevented before occurs.